This application is based on Japanese Patent Application No. 2000-116699, No. 2000-188785, and No. 2001-94226 with domestic priority claimed from the former two applications, the content of which are hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to an electrode for a high pressure discharge lamp, a high pressure discharge lamp, and a method of manufacturing therefor.
(2) Description of Related Art
In recent years there has been active development of projection type image display apparatuses such as liquid crystal projectors. In such a projection type image display apparatus it is necessary to have a high intensity light source close to a point light source. Generally a high pressure discharge lamp such as a high pressure mercury lamp or a metal halide lamp of the short arc type is used as this kind of light source.
One of the main technical tasks when developing high pressure discharge lamps of the short arc type is lengthening the life by improving the life characteristics. Namely, generally in high pressure discharge lamps of the short arc type, the tungsten which forms the electrode melts and disperses, the electrode tip becomes deformed and wears due to the temperature of the electrode end increasing excessively, while the dispersed tungsten is deposited on the inner surface of the light-emitting tube, causing blackening. This blackening of the inner surface of the light-emitting tube causes premature degradation of light flux. In order to solve this problem, conventionally various techniques have been investigated relating to design of electrodes for high pressure discharge lamps of the short arc type and manufacturing methods of the electrodes.
As prior art relating to the above described electrode design, an electrode which has a construction such as that shown in FIG. 1 has been developed. The electrode 901 shown in FIG. 1 is formed by an electrode rod 902 with a narrow shaft diameter, and a cylindrical electrode part 903 whose inside diameter is larger than the electrode rod 902, in combination. The characteristics of the operation of the electrode are (1) the cylindrical electrode part 903 lowers the temperature of the electrode tip 904 by transferring heat generated therein rapidly to the electrode rod side, suppressing deformation and wear of the electrode tip 904 by melting and dispersion of the electrode metal, and (2) through the working of the electrode rod 902 with a narrow shaft diameter, the whole of the electrode 901 is thermally insulated, promoting the evaporation of light emitting material enclosed in the light-emitting tube.
An electrode such as the electrode 901 is ordinarily manufactured by a grinding process of a block of a high melting point metal material such as tungsten, and is used as an anode in particular in high pressure discharge lamps of the short arc type such as super high pressure mercury lamps and high pressure xenon lamps of the DC discharge type which are subject to high rises in temperature.
Meanwhile, initially electrodes of the same construction as high pressure discharge lamps used for general lighting of the long arc type were used for metal halide lamps and high pressure mercury lamps of the short arc type which are used as light sources for projection type image display apparatuses of recent years. As shown in FIG. 2, an electrode 911 is formed by an electrode rod 912 made from ordinary tungsten, and a coil 913 of tungsten wire which has a narrow wire diameter. However, in a high pressure discharge lamp of the short arc type which uses an electrode such as the electrode 911, the above-described deformation and wear of the electrode tip due to melting and dispersion of the tungsten electrode material cannot be avoided, making lengthening the life of the lamp difficult.
Subsequently, as a way of solving the problem of lengthening the life of such a lamp, electrodes which have the basic structure shown in FIG. 1 which were developed for use in conventional high pressure discharge lamps of the short arc type were re-investigated. However, as it is costly to manufacture electrodes by a grinding process, an electrode that can be manufactured cheaply while having the same basic construction as the electrode 901 in FIG. 1 was investigated. Prior art relating to such electrodes is disclosed, for example, in Japanese Patent Number 2820864 and Japanese Patent Laid-Open No. H10-92377.
Examples of the electrodes of the above-described patents are shown in FIG. 3A and FIG. 3B. An electrode 921 is manufactured through two processes which are simple compared to the above-described grinding process: (a) first, a tungsten wire coil 923 is wound and set around the discharge end of the tungsten electrode rod 922 (see FIG. 3A), and (b) the discharge side end of the electrode rod 922 and the discharge side end of the coil 923 are melted and fused by a so-called electric discharging method to form an electrode tip 924 which is substantially a semi-sphere (see FIG. 3B).
In the electrode 921 the section formed by the coil 923 and the semi-spherical electrode tip 924 has the same effect as the cylindrical electrode part 903 and the electrode tip 904 of the electrode 901 shown in FIG. 1. Consequently, the heat in the semi-spherical electrode tip 924 is transferred rapidly to the coil 923, lowering the temperature of the electrode tip 924. In this way, even electrodes manufactured using low cost manufacturing electric discharging methods, melting and dispersion of the electrode material and deformation and wear of the electrode tip can be suppressed and life can be lengthened.
Please note that another piece of prior art relating to improving life expectancy of high pressure discharge lamps is a means which uses tungsten of high purity as an electrode material, disclosed in Japanese Patent Laid-Open No. H9-165641. Here, a result is shown that using tungsten of high purity in which the sum total of the elements of the accessory constituents Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Si, Sn, Na, K, Mo, U and Th is regulated to 10 ppm of the principal component tungsten W is used as the electrode (particularly the anode) material in large discharge lamps with high output is effective in improving lamp electrode life span.
Based on the above-described related art, the present inventors worked toward developing a high pressure mercury lamp of the short arc type which can be used as a light source in projection type image display apparatuses. In the development the inventors set two objectives which relate in particular to the performance of lamps demanded by the market. The objectives were (1) making the distance between the electrodes, in other words, the distance between the discharge ends of the two electrodes provided in opposition in the light-emitting tube, no more than 1.5 mm, which is shorter than conventional spacing, in order to improve light usage efficiency when combined with a reflective mirror, and (2) to accomplish a lamp life expectancy of at least 3000 hours. Please note that (2) lamp life expectancy, as will be explained below, is defined by the aging time when the light flux maintaining rate estimated from the average illuminance maintaining rate of nine points on a screen during light emission by the lamp unit drops to 50%.
The present inventors, when beginning development, investigated developing a high pressure discharge lamp of the short arc type which has shorter distance between electrodes than conventional lamps, using electrodes made by an electric discharging method based on the methods in the above-described patents (FIGS. 3A and 3B). However, when the inventors measured characteristics of mass produced lamps which use such electrodes, they discovered much variation between lamps in characteristics such as voltage and life, meaning such lamps lack commercial viability.
Subsequently, when the cause of the above-described variations in lamp characteristics was investigated, it was revealed that the fused shapes of the electrode ends manufactured with the conventional electrical discharging method were not uniform semi-spheres, but rather various shapes and dimensions had been produced, and these various shapes and dimensions where the cause of the variation in lamp characteristics. For example, when the shape of the electrode tip was not semi-spherical, there were cases in which the discharge arc deviated from the center axis between the two electrodes. As a result the length of the discharge arc was longer than the design value, therefore the lamp voltage increased beyond the rating value range.
In particular, when the distance between electrodes is in the range of the inventors"" objective of 1.5 mm or less, it was clear that fluctuations in lamp voltage according to this kind of variation in the length of the in discharge arc increase. Furthermore, when there are variations in the fused shape and the dimensions of electrode tips between lamps, the temperature of the electrode tips during discharge differs, giving rise to variations in the life of the lamps.
The object of the present invention is to provide a high pressure discharge lamp, a high pressure discharge lamp electrode and a manufacturing method therefor which achieves desirably a life of at least 3000 hours, and can suppress variations in lamp characteristics in a high pressure discharge lamp which uses an electrode of which the discharge side tip has been fused.
The above-described objective can be achieved by a method of manufacturing for a high pressure discharge lamp which includes a covering member applying step for applying a covering member made of refractory metal on a discharge side end of an electrode rod made of refractory metal so as to cover a circumference of the electrode rod in a vicinity of the discharge side end, and a fusing step for integrating the discharge side end into a semi-sphere by intermittently heat fusing the discharge side end on which the covering member is applied.
In this method of manufacturing, temperature of the electrode tip can easily be controlled in the electrode manufacturing process due to the discharge side tip of the electrode being heat fused intermittently. According to this method, variations in, for instance, shape of the electrode tip can be suppressed, more specifically, it is possible to form the electrode tip into a semi-sphere without causing internal holes for instance. Therefore, lengthening of the life of the lamp is achieved together with variations in lamp characteristics being suppressed.
Please note that by performing heat fusing intermittently the size of the average grain diameter in the crystallization of the electrode tip can be increased. Thus, for example, the above-described objective can be achieved by a high pressure discharge lamp including electrodes which are made of a material having tungsten as a main constituent and are placed in a light-emitting tube so that semi-sphere ends are in opposition, and an average grain diameter in tungsten crystallization of the electrode end being at least 100 xcexcm. Deformities in the electrode can be suppressed due to the heat capacity increasing in the electrode tip of this kind of electrode whose average grain diameter in crystallization is large, contributing to lengthening the life of the high pressure discharge lamp.
Please note that as a specific method for the above-described intermittent heat fusing, the present inventors found that, for example, a method using discharge arc fusing or a method using a laser is particularly desirable.